Closing of glass packaging containers, more particularly in dairying, is generally obtained by crimping aluminum lids, said lids being crimped or wrapped on the rim of the mouth of containers such as bottles in the milk industry or yogurt cups.
Such a crimping is not tight and care must be observed when handling the containers. Moreover, for alimentary foods such as milk, yogurt or cream, said crimped closing is not air-tight and accordingly the product contained within the container is not prevented from contact with the surrounding atmosphere. Such contact promotes undue fermentation and oxidation of said product.
Increased development of thermoplastic containers has made it possible to close such containers by fusion welding or sealed bonding by making use of composite lids comprised by a foil of aluminum and a layer of a thermoweldable plastic material. When applying the closing head of a closing tool onto the mouth of such containers, there is automatically obtained a thorough contact between the lid and the end surface of the mouth even if said surface is not previously exactly flat owing to the ability of the plastic material of the container to be crushed and to be creeped when heated by the closing head.
The utilization of such techniques for sealing glass containers raises difficulties in that said containers, which are generally made by molding, have uncontrolled local distortions, when cooled out of the mold, especially in the mouth area thereof. It results therefrom that the upper or outer portion of the mouth is not exactly flat and generally extends substantially in a plane which is not parallel to the base of the container. The upper rim of the mouth generally shows noticeable buckling with local dimensional variations up to about 3/10 of a millimeter.
Due to said irregular shape of the mouth of glass containers, it is not possible, with conventional closing apparatus, to obtain a sealed closing of the containers since sealed closing requires a perfect contact between the lid and the entire contact surface of the mouth.
In order to avoid such drawbacks, it has been proposed to make use of closing devices embodying a cushioning pad made from a resilient material capable of accomodating local level variations of the container mouth and adapted for transmitting heat and applying a force which is enough for achieving heat sealing of the container in the considered temperature conditions.
However, said devices show difficulties in conveniently transmitting heat through said distortable cushioning pad due to the lower thermal conductivity of the material of said pad. The addition in said material of substances showing good thermal conductivity results generally in an increased rigidity of the pad which is detrimental for the heat sealing or closing of relatively fragile containers, for example those of the non-return type such as those for packaging liquid or semi-liquid alimentary products. With such closing devices, it is difficult to achieve a balance between the desired flexibility of the pad in order not to require excessive high pressure conditions in the closing operation prone to injure, e.g. break the containers, and desired convenient heat conductivity to avoid excessively high power sources for generating heat.